“I highly suspect that, with the amount of coverage that this incident is getting, there are probably already people that are working to incorporate the exploit that was used for spreading,” he said.

Investigators are working to track down those responsible for the ransomware used on Friday, known as Wanna Decryptor or WannaCry.

‘Bring them to justice’

The virus exploits a vulnerability in Microsoft Windows software, first identified by the US National Security Agency, experts have said.

Europol described the cyber-attack as “unprecedented” and said its cyber-crime team was working with affected countries to “mitigate the threat and assist victims”.

Oliver Gower, of the UK’s National Crime Agency, added: “Cyber criminals may believe they are anonymous, but we will use all the tools at our disposal to bring them to justice.”

Update not applied

In the UK, critics said the government had known about the threat of a cyber-attack for some time, but hospitals had not made the right upgrades to protect themselves.

A security update – or patch – was released by Microsoft in March to protect against the virus, but it appears many NHS organisations had not applied it or were using an older version of the operating system no longer supported – namely Windows XP.

Kingsley Manning, a former chairman of NHS Digital, claimed that several hundred thousand computers were still running the out-of-date operating system.